The field of the present invention relates to data reading systems. In particular, a method and apparatus are described herein for detecting objects with an optical reading unit, such as a bar code scanner.
Whether stationary, handheld, or combination stationary/handheld, data reading devices are useful for reading a wide array of indicia found on consumer and industrial products, such as traditional linear or one-dimensional bar codes, two-dimensional symbologies, matrix array symbols, patterns and logos, signatures, and other images. Information encoded in any number of different encoded formats may then be used for numerous applications including retail check-out, inventory management, material handling, manufacturing, and package sortation and transportation.
Current data readers may incorporate techniques for automatically detecting an object and initiating and terminating scanning and decoding operations. For example, a clerk at a check-out line using a handheld scanner that incorporates today""s automatic reading techniques now has the option of mounting the scanner in a stand, which advantageously provides hands-free operation, or manually actuating the trigger to begin the data acquisition process.
Despite their apparent advantages, current automated techniques for object detection, as well as symbol scanning and decoding, have their drawbacks. Most techniques for object detection require the use of one or more additional circuits, such as the active and passive circuits described in, for example, U.S. Pat. No. 5,525,789 issued to Rockstein et al. Such circuits typically contain additional components, which may include an infra-red light source, an ultrasonic energy source, focusing lenses, and/or separate photodiodes such as those described in the foregoing patent, that increase the expense, size, and maintenance costs of the data reader.
Moreover, current object detection techniques are typically ineffective. Falsely detecting objects and/or target indicia is a common problem with current object detection techniques as they fail to take into account the random noise inherent within a scanner and the fundamental principle that surface properties of target materials will exhibit varying degrees of reflectivity. For example, U.S. Pat. No. 5,525,789 describes a common technique which determines whether a target is detected by comparing a received signal to an emitted signal within a fixed time limit. A preset time limit or fixed trigger threshold, however, inaccurately assumes a scanner does not produce random noise inherent within the circuitry caused by part tolerances, temperature, etc., and inaccurately assumes that all surface properties of target materials have the same degree of reflectivity. To the contrary, random noise and varying degrees of reflectivity may cause a spatial time variance between an emitted signal and a received signal and this frequency range response cannot be accommodated with a fixed time limit or a fixed trigger threshold. Assuming otherwise will produce trigger sensitivity problems such as a decreased trigger point distance or a continual re-triggering of the scanner if the fixed threshold is improperly set. Although these problems can be minimized or reduced by resetting the fixed time limit or fixed trigger threshold, such a modification is very cumbersome for the user.
An improved automated technique that provides for a good deal of immunity against falsely detecting targets due to random noise within the scanner""s circuitry is disclosed in U.S. Pat. No. 5,260,554 issued to Grodevant, which patent is incorporated by reference as if fully set forth herein. This patent discloses a technique of pulsing a light beam toward a reflector, such as reflective tape adhered to the base of a stand used to support the scanning unit. The presence of an object is detected when the number of emitted pulses exceeds two times the number of received pulses during a given time period. The present inventors have recognized the need for an improved system for detecting objects with an optical reader that is real-time responsive to random noise within a scanner""s circuitry, accommodates varying degrees of reflectivity of target materials, and requires neither additional circuitry nor reflective tape to accurately detect an object.
The preferred embodiments relate to a system and method for detecting an object within the field of view of an optical reader, such as bar code scanner. Object detection is determined by a software algorithm that may be embedded in a hardware component of the scanner, such as a controller or microprocessor. The object detection software algorithm uses a current trigger threshold to determine whether there is a change in the amount of reflected light energy sufficient to indicate the presence of motion in the scanner field of view.
In a preferred embodiment, a current trigger threshold is set by pulsing a laser diode, generating a signal of returned light energy, sampling the signal until a transition in the signal occurs, and measuring the amount of time from when the pulse stopped to when the transition occurred. The count value of the current trigger threshold is then preferably deviated by a configurable amount to obtain a current trigger threshold range. A current sample average may then be compared to the current trigger threshold range in order to detect a trigger condition and enable the system for scanning and decoding.
A scanner operating in accordance with the above-described method, may include a scan engine having a light source capable of being modulated, an optical interface for receiving information, and a processor for conditioning the received information and determining whether an object is detected. Advantageously, the effective range of a scanner utilizing the above-described method is increased as a scan event will be triggered based on a relatively small time change. Moreover, the above-described method normalizes random noise within a scanner without additional circuitry or reflective tape. Various other embodiments may utilize some but not all of the above elements, or may include additional refinements, while obtaining the benefit of accurate object detection.